North American Ice Sheet Dwindled Fast in Conditions Like Today's

In the face of warming climate, researchers have yet to agree on how much and how quickly melting of the Greenland ice sheet may contribute to sea level rise. To shed light on this question, scientists at the University of Wisconsin and Columbia University’s Center for Climate Systems Research (CCSR) analyzed the disappearance of the Laurentide ice sheet, which covered much of North America during the last ice age. The findings, published online in Nature Geoscience, suggest that such melting can happen very rapidly, and could begin by the end of this century.

On the premise that the Laurentide’s fate is the closest analogy for what may happen in Greenland, the team analyzed geologic data and computer models to reconstruct its demise. Until about 18,000 years ago, the ice covered most of Canada and the upper reaches of the United States, until it pulled back and finally disappeared around 6,500 years ago. It saw two particularly rapid intervals of melting: the first around 9,000 years ago, and the second 7,500 years ago. The researchers’ model estimates that the first phase of rapid retreat led to about 7 meters of sea level rise, at a rate of about 1.3 centimeters a year. The second phase accounted for 5 meters of sea rise--about 1 centimeter a year. These rates match evidence derived from corals on the seafloor, as well as organic matter in sediments on land.

“We have never seen an ice sheet retreat significantly or disappear before,” said Anders Carlson, the study’s lead author and assistant professor of geology and geophysics at the University of Wisconsin-Madison. “Yet this may happen for the Greenland ice sheet in the coming centuries to millennia. What we don’t know is the rate. This gives us a window into how fast these large blocks of ice can melt and raise sea level.”

The simulations of the melting suggest that the driving factor was increased solar radiation caused by a change in the earth’s orbit, which in turn increased summer temperatures. Given current warming trends linked to human activities, it is thought that similar temperature rises may occur over Greenland by 2100.

The estimated current rate of sea-level rise worldwide is about 3 millimeters per year, but in a report issued last year, the Intergovernmental Panel on Climate Change (IPCC) predicted that seas could rise 59 centimeters by the end of this century—more, if global temperatures rise faster than projected. However, the IPCC predictions are based mainly on the expansion of the oceans through warming, accounting less for contributions from melting of the great ice sheets in Greenland and Antarctica. (Greenland’s covers 1.7 million kilometers.) The new analysis suggests that the pressures on the Laurentide ice sheet 9,000 years ago were similar to those that Greenland will face by the year 2100. This implies a potential for melting substantially greater than the IPCC projections.

“The word ‘glacial’ used to imply that something was very slow,” said LeGrande. “This new evidence compiled from the past paired with our model for predicting future climate indicates that ‘glacial’ is anything but slow. Past ice sheets responded quickly to changing climate. That hints at the potential for a similar response in the future.”

In an accompanying commentary that appeared with the report, Lamont-Doherty scientists Mark Siddall and Michael Kaplan wrote that the Greenland ice mass seems to be entering a period when it is “particularly vulnerable.” Siddall and Kaplan said, “Their work suggests that future reductions of the Greenland ice sheet on the order of one meter per century are not out of the question.”